It's especially true of in-ceiling speakers.  There's just something lacking about a speaker that's firing straight down.  But QPD's new line of motorized speakers solves the sound issue without sacrificing the look.

Take the QPD 2006 for example.  It's a 2-way speaker that boasts a 1 inch tweeter and a 6.5 inch woofer.  When not in use, it can sit flush against the wall or ceiling like traditional hidden speakers.  But it can also kick out to an angle of 25, 30, 40 or 50 degrees.  

They're incredibly cool but they're not, strictly speaking, invisible.  When science finally advances to the point that we can create actual invisible speakers, we're going to have to think of a new name for low-profile in-ceiling speakers.